Sylvanas’ wedge was supposed to be Genn, and pressed by the Horde holding Teldrassil, as she explains in A Good War:
“It will take years before they can even consider retaking Darnassus.”
“You understand, High Overlord,” Sylvanas said. “Think it through. What happens next?”
“They might try to conquer the Undercity . . . but Darnassus becomes our hostage against that. The night elves will not allow your city to fall if they fear it means you will destroy theirs . The same goes for a strike against Silvermoon.” Saurfang’s thoughts raced. She’s right. This could work. “And even if the Alliance agrees to retake Darnassus . . . The Gilneans!”
Sylvanas’s eyes disappeared beneath the edge of her hood. “They lost their nation years ago. The Gilneans will be furious if the Alliance acts to help the kaldorei first,” she said. “The boy in Stormwind will have a political crisis on his hands. He is smart, but he is not experienced. What happens when Genn Greymane, Malfurion Stormrage, and Tyrande Whisperwind all demand differing actions? He is not a high king like his father. The respect the others give him is a courtesy, not an obligation. Anduin Wrynn will rapidly become a leader who cannot act. If the Alliance will not march as one, each nation will act in its own interest. Each army will return home to protect their lands from us.”
Tyrande alone doesn’t act as a wedge without Genn betraying the Night Elves, which Genn never would have done any way. Genn in fact subverts Tyrande being a wedge even when she does leave for Darkshore without Anduin’s support by supporting her instead like he said he would in Elegy, in the quest “Shores of Fate”:
Genn Greymane says: Anduin... I need a word.
Anduin Wrynn says: Of course. You know you can always speak your mind.
Genn Greymane says: The night elves saved my people from our curse. They offered us refuge in Darnassus after our kingdom fell.
Genn Greymane says: I cannot stand idle as they endure the same fate we did. Gilneas will fight by their side.
Genn Greymane says: I don't mean to defy you. But if I didn't give the order, I think Mia would charge off to battle without me.
Anduin Wrynn says: I understand, Genn. Light be with you.
And on top of that, Anduin spins everything as it working out in the best interest of the Alliance in the end any way, between the quests “Waning Hope” and “Shores of Fate”:
Sira Moonwarden says: Tyrande left for Darkshore ahead of our army. She intends to wrest it from the Horde herself.
Anduin Wrynn says: I see.
Anduin Wrynn says: <Name>, our forces in Zuldazar have not yet begun their assault. With your aid, we may be able to succeed on both fronts.
Maiev Shadowsong says: Your aid is welcome, <name>. At least we can count on you.
Which ultimately it does, as we find out after the Battle of Dazar’alor from Shaw and Nathanos.
TL;DR: Neither killing Malfurion or burning Teldrassil would have made Tyrande into a wedge that would split the Alliance because Genn never would have betrayed the Night Elves.